Fix the crash that could occur when the window is closed while web contents drag is in progress.

content/browser/web_contents/web_drag_source_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "content/browser/web_contents/web_drag_source_mac.h"
 
 #include <sys/param.h>
 
 #include "base/bind.h"
 #include "base/file_path.h"
@@ skipping 117 matching lines @@
     dragOperationMask_ = dragOperationMask;
 
     fileExtension_ = nil;
 
     [self fillPasteboard];
   }
 
   return self;
 }
 
+- (void)clearWebContentsView {
+  contents_ = nil;
+  contentsView_ = nil;
+}
+
 - (NSDragOperation)draggingSourceOperationMaskForLocal:(BOOL)isLocal {
   return dragOperationMask_;
 }
 
 - (void)lazyWriteToPasteboard:(NSPasteboard*)pboard forType:(NSString*)type {
   // NSHTMLPboardType requires the character set to be declared. Otherwise, it
   // assumes US-ASCII. Awesome.
   const string16 kHtmlHeader = ASCIIToUTF16(
       "<meta http-equiv=\"Content-Type\" content=\"text/html;charset=UTF-8\">");
 
@@ skipping 113 matching lines @@
                  at:position
              offset:NSZeroSize
               event:dragEvent
          pasteboard:pasteboard_
              source:contentsView_
           slideBack:YES];
 }
 
 - (void)endDragAt:(NSPoint)screenPoint
         operation:(NSDragOperation)operation {
+  if (!contents_)
+    return;
   contents_->SystemDragEnded();
 
   RenderViewHostImpl* rvh = static_cast<RenderViewHostImpl*>(
       contents_->GetRenderViewHost());
   if (rvh) {
     // Convert |screenPoint| to view coordinates and flip it.
     NSPoint localPoint = NSMakePoint(0, 0);
     if ([contentsView_ window])
       localPoint = [self convertScreenPoint:screenPoint];
     NSRect viewFrame = [contentsView_ frame];
@@ skipping 11 matching lines @@
     rvh->DragSourceEndedAt(localPoint.x, localPoint.y,
                            screenPoint.x, screenPoint.y,
                            static_cast<WebKit::WebDragOperation>(operation));
   }
 
   // Make sure the pasteboard owner isn't us.
   [pasteboard_ declareTypes:[NSArray array] owner:nil];
 }
 
 - (void)moveDragTo:(NSPoint)screenPoint {
+  if (!contents_)
+    return;
   RenderViewHostImpl* rvh = static_cast<RenderViewHostImpl*>(
       contents_->GetRenderViewHost());
   if (rvh) {
     // Convert |screenPoint| to view coordinates and flip it.
     NSPoint localPoint = NSMakePoint(0, 0);
     if ([contentsView_ window])
       localPoint = [self convertScreenPoint:screenPoint];
     NSRect viewFrame = [contentsView_ frame];
     localPoint.y = viewFrame.size.height - localPoint.y;
     // Flip |screenPoint|.
@@ skipping 143 matching lines @@
 - (NSImage*)dragImage {
   if (dragImage_)
     return dragImage_;
 
   // Default to returning a generic image.
   return content::GetContentClient()->GetNativeImageNamed(
       IDR_DEFAULT_FAVICON).ToNSImage();
 }
 
 @end  // @implementation WebDragSource (Private)